The present invention relates to time-of-flight mass spectrometers.
In time-of-flight mass spectrometers (TOFMS), the sample to be analyzed is ionized, accelerated in a vacuum through a known potential, and then the arrival time of the different ionized components is measured at a detector. The larger the particle, the longer the flight time; the relationship between the flight time and the mass can be written in the form:
time=k{square root over (m)}+c
where k is a constant related to flight path and ion energy, c is a small delay time, which may be introduced by the signal cable and/or detection electronics.
The detector converts ion impacts into electrons. The signal generated by the detector at any given time is proportional to the number of electrons. There is only a statistical correlation between one ion hitting the detector and the number of electrons generated. In addition, more than one ion at a time may hit the detector due to ion abundance.
The mass spectrum generated by the spectrometer is the summed output of the detector as a function of the time-of-flight between the ion source and the detector. The number of electrons leaving the detector in a given time interval is converted to a voltage that is digitized by an analog-to-digital converter (ADC). The dynamic range of the detector output determines the required number of ADC bits.
A mass spectrum is a graph of the output of the detector as a function of the time taken by the ions to reach the detector. In general, a short pulse of ions from an ion source is accelerated through a known voltage. Upon leaving the accelerator, the ions are bunched together but travelling at different speeds. The time required for each ion to reach the detector depends on its speed, which in turn, depends on its mass.
A mass spectrum is generated by measuring the output of the ADC as a function of the time after the ions have been accelerated. The range of delay times is divided into discrete xe2x80x9cbinsxe2x80x9d. Unfortunately, the statistical accuracy obtained from the ions that are available in a single such pulse is insufficient. In addition, there are a number of sources of noise in the system that result in detector output even in the absence of an ion striking the detector. Hence, the measurement is repeated a number of times and the individual mass spectra are summed to provide a final result having the desired statistical accuracy and signal to noise ratio.
There are two basic models for generating the mass spectrum. In the first model, the output from the detector is monitored for a pulse indicative of an ion striking the detector. When such a pulse is detected, the value of the detector output and the time delay associated with the pulse are stored in a memory. Such xe2x80x9ceventxe2x80x9d spectrometers require less memory to store a spectrum since only the peaks are stored.
The second type of spectrometer avoids this discrimination problem by measuring the output of the detector on every clock pulse after the ions have been accelerated and summing the data even if it is likely to be noise. Since no data is discarded, such xe2x80x9csummedxe2x80x9d spectrometers can measure peaks that only appear above the background after a large number of scans are added together.
The resolution of the spectrometer depends on the number of bins into which the flight time measurements are divided. As the number of bins is increased, the rate with which the output of the detector is sampled also increases and the signal-to-noise ratio decreases.
If the TOFMS has a noise level that is less than 1 ADC least significant bit (LSB) and a signal that is greater than 1 ADC LSB, a fine adjustment to the DC offset of the signal can be made such that the noise falls within ADC count 0 and 1. This assures that the signal sums, while the noise that occurs on the baseline does not.
As the sample rate is increased, a point is reached at which the noise is no longer less than the ADC LSB. To take advantage of faster sample rates, the analog bandwidth of the pre-amp and the input of the ADC are increased proportionally. Since noise increases as the square root of the bandwidth, faster sampling rates introduce more noise into the output data. In addition, ADCs that are optimized for high frequency signals may have increased noise when DC background signals are digitized.
Broadly, it is the object of the present invention to provide an improved TOFMS.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a summed TOFMS having a filter for identifying detector outputs that are likely to be the result of noise rather than ions striking the ion detector. The TOFMS includes an ion accelerator for generating an ion pulse in response to a start signal. A clock generates a series of clock pulses that are used to increment a register value. The TOFMS stores a plurality of data values in a memory at locations specified by the register value. The filter receives the ion measurements from the ion detector and generates an output measurement value corresponding to each ion measurement. The filter sets the output measurement value to a predetermined baseline value if the filter determines that the ion measurement is noise, otherwise, the filter sets the output measurement value to the ion measurement. An adder, responsive to the clock signal, forms the sum of the data value addressed by the register value and the output measurement value and stores the sum in the memory at the location corresponding to the register value. In one embodiment, the filter determines that one of the ion measurements is noise if the ion measurement is within a pre-assigned threshold value of the baseline. In another embodiment, the filter determines that one of the ion measurements is not noise if the measurement is greater than a first threshold and a function of the ion measurements corresponding to a predetermined number of adjacent register values is greater than a second threshold. In another embodiment, the function is the value generated by a finite impulse response filter operating on a sequence of the ion measurements.